1. Field of the Invention
The present invention relates to an electrode structure of an electron gun for a cathode ray tube, and more particularly to an electrode structure of an electron gun for a cathode ray tube which can improve the resolution of a cathode ray tube by providing a dipole lens, removing a halo portion and the horizontal lengthening of a beam spot due to increase the amount of deflection and the focal distance in the peripheral portion of a screen of the cathode ray tube.
2. Description of the Prior Art
An electron gun for a cathode ray tube is generally formed by fixing a plurality of grid electrodes having circular electron beam-passing apertures so as to be in-line at specific intervals by way of bead glasses.
Referring to FIG. 1, a conventional electrode structure of an electron gun for an in-line type color picture tube comprises three cathodes K.sub.1, K.sub.2 and K.sub.3 respectively having each built-in heaters H.sub.1, H.sub.2 and H.sub.3 for emitting thermions, a first grid electrode G.sub.1 and a second grid electrode G.sub.2 for controlling and accelerating thermions to form an electron beam, and a first accelerator and focus electrode G.sub.3, a second accelerator and focus electrode G.sub.4, and an anode G.sub.5 constituting a main focus lens for forming a beam spot on the screen by further focussing the electron beam.
Also, a multistage focus type electron gun which strengthens a focus effect further comprises a third grid electrode and a fourth grid electrode for reserve focus between a grid electrode which forms an electron beam and an accelerator and focus electrode which constitutes a main focus lens.
In such a cathode ray tube, it is preferably that a beam spot is uniform over the screen in order to obtain high picture quality.
However, the electron beam from an electron gun is deflected to be projected over the screen according to the intensity of deflection magnetic field of deflection yoke provided near the outlet of the electron gun. Accordingly, a small circular beam spot can be formed in the central portion of the screen on which a deflection magnetic field has no effect, but the beam spot is lengthened in the horizontal direction in the peripheral portion of the screen shown in dotted line in FIG. 8.
A beam spot lengthened in the horizontal direction as above consists of a core portion whose electron density is high and a halo portion whose electron density is low. The above horizontal lengthening of a beam spot becomes significant in the peripheral portion of the screen because a self-convergence deflection magnetic field becomes more intensive and the distance from the screen becomes longer as the electron beam goes to the peripheral portion of the screen, thereby causing deterioration in the resolution of the screen.
In order to remove the above phenomenon, a dynamic voltage compensating for the above phenomenon is previously applied to one of the electrodes. In U.S. Pat. No. 4,772,827 to Kuniharu Osakabe, there is disclosed an electron gun comprising a rear focus electrode system which includes a first focus electrode, a second focus electrode comprising a first grid electrode and a second grid electrode, and a third grid electrode disposed between the first and second focus electrodes. And, the electron gun of Kuniharu Osakabe applies a constant focus voltage to the first focus electrode and the first grid electrode applies to the second grid electrode a dynamic voltage superimposed on the constant voltage and applies a high voltage to the third grid electrode system.
However, according to the invention of Kuniharu Osakabe, two grid electrodes are added and a quadruple lens is formed between the two grid electrodes by way of plate shaped projections attached to the surface of the grid electrodes. Therefore, the electrode structure becomes complex and the manufacturing cost increases.